To come back to the original question: After the upgrade to Windows 10, you have a 30 day period to go back to the previous version of Windows. The question now was whether - after these 30 days were over (and the previous version had been removed from the PC) and if you then went back to the previous version, if then this old version would still (meaning: again) activate.
The answer is: Yes, the old version of Windows will activate again. The old key can stil be used for the old version:
If you have installation media and a product key for your previous version of Windows, you can use them to go back. Get the details in the Use installation media to reinstall Windows section of this topic.